Download Remote Procedure Call

• Layered protocols
• Remote Procedure Call (RPC)
• Issues:
– Parameter passing
– Binding
– Failure handling
– Performance and implementation issues
CS550: Advanced Operating Systems
2
• Protocols are agreements/rules on communication
• Protocols could be connection-oriented or
connectionless
CS550: Advanced Operating Systems
3
• A typical message as it appears on the network.
2-2
CS550: Advanced Operating Systems
4
• Goal: ?
• Sample Issues:
–
–
–
–
how to encode a 0 Vs. 1?
what voltage should be used?
how long does a bit need to be signaled?
what does the cable, plug, antenna, etc. look like?
• Examples:
– modems
– “knock once for yes, twice for no”
– X.21
CS550: Advanced Operating Systems
5
• Goal: ?
• Sample Issues:
– how big is a frame? (framing)
– can I detect an error in sending the frame? (error
control)
– what demarks the end of the frame?
– how to control access to a shared channel? (flow
control)
• Examples:
– Ethernet framing, Serial line IP (SLIP), Point-to-point
protocol (PPP)
CS550: Advanced Operating Systems
6
• Goal: ?
• Sample Issues:
– how to route packets that have to travel several
hops?
– Congestion control algorithm: traffic shaping, flow
specifications, and bandwidth reservation
– accounting - charge for use of the network
– fragment or combine packets depending on rules
of link layer
• Examples:
– IP
CS550: Advanced Operating Systems
7
• Goal: ?
• Sample Issues:
– how to order messages and detect duplicates
– error detection (corrupt packets) and
retransmission
– connectionless or connection-oriented
• Examples:
– TCP (transmission control protocol)
– UDP (universal datagram protocol)
CS550: Advanced Operating Systems
8
• Goal: ?
• Sample Issues:
– Session layer: Allows users on different machines
to establish sessions between them; Provides
dialog control, to keep track of which party is
currently talking, and synchronize
– Presentation layer: Encodes data in a standard
agreed upon way; Allows users to insert
checkpoints into long transfer
• Examples:
– eXternal Data Representation (XDR)
CS550: Advanced Operating Systems
9
• Goal: ?
• Sample Issues:
– when sending email, what demarks the subject
field
– how to represent cursor movement in a terminal
• Examples:
– Simple Mail Transport Protocol (SMTP), File Transfer Protocol
(FTP), Hyper-Text Transport Protocol (HTTP), Simple Network
Management Protocol (SNMP), Network File System (NFS),
Network Time Protocol (NTP), Net News Transport Protocol
(NNTP), X (X Window Protocol)
CS550: Advanced Operating Systems
10
• Middleware:
– An application that logically lives in the application layer
– Contains many general-purpose protocols that warrant their
own layers
CS550: Advanced Operating Systems
11
• Client-Server provides a mechanism for
services in distributed systems BUT
–requires explicit communication (sendreceive)
• Q: How do me make “distributed
computing look like traditional
(centralized) computing”?
• Can we use procedure calls?
CS550: Advanced Operating Systems
12
• In Distributed systems: the callee may be
on a different system
–Remote Procedure Call (RPC)
–NO EXPLICIT MESSAGE PASSING
• Goal: Make RPC look like local
procedure call
CS550: Advanced Operating Systems
13
• Parameter passing
• Binding
• Reliability/How to handle failures
–messages losses
–client crash
–server crash
• Performance and implementation issues
• Exception handling
• Interface definition
CS550: Advanced Operating Systems
14
count=read(fd,buf,nbytes);
a)
Parameter passing in a
b) The stack while the called
local procedure call: the
procedure is active
stack before the call to
CS550: Advanced Operating Systems
read
15
• Parameters (in C):
–call-by-reference OR call-by-value
• Value parameter (e.g., fd, nbytes)
• Reference parameter (array buf)
• Many options are language dependent
CS550: Advanced Operating Systems
16
• Local procedure parameter passing
– Call-by-value
– Call-by-reference
• Remote procedure calls simulate this
through:
– Stubs – proxies
– Marshaling
• How about global variables?
CS550: Advanced Operating Systems
17
• Client makes procedure call (just like a
local procedure call) to the client stub
• Server is written as a standard procedure
• Stubs take care of packaging arguments
and sending messages
• Packaging is called marshaling
• Stub compiler generates stub
automatically from specs in an Interface
Definition Language (IDL)
CS550: Advanced Operating Systems
18
• Principle of RPC between a client and server program.
CS550: Advanced Operating Systems
19
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Client procedure calls client stub in normal way
Client stub builds message, calls local OS
Client's OS sends message to remote OS
Remote OS gives message to server stub
Server stub unpacks parameters, calls server
Server does work, returns result to the stub
Server stub packs it in message, calls local OS
Server's OS sends message to client's OS
Client's OS gives message to client stub
Stub unpacks result, returns to client
CS550: Advanced Operating Systems
20
• Steps involved in doing remote computation through
RPC
CS550: Advanced Operating Systems
21
• Problem: different machines have
different data formats
– Intel: little endian, SPARC: big endian
• Solution: ?
CS550: Advanced Operating Systems
22
• Problem: how do we pass pointers?
– If it points to a well-defined data structure,
?
– What about data structures containing
pointers?
• Marshalling: transform
parameters/results into a byte stream
CS550: Advanced Operating Systems
23
• Problem: how does a client locate a
server?
• Binder can be a bottleneck
• Binder can do load balancing
CS550: Advanced Operating Systems
24
• Client unable to locate server: return error
• Lost request messages: simple timeout mechanisms
• Lost replies: timeout mechanisms
– Make operation idempotent
– Use sequence numbers, mark retransmissions
• Server failures: did failure occur before or after
operation?
–
–
–
–
At least once semantics (SUNRPC)
At most once
No guarantee
Exactly once: desirable but difficult to achieve
• Client failures
CS550: Advanced Operating Systems
25
• Reasons:
–server may be down
–new version of server but older client
• Solutions
–respond with error type “cannot locate
server”
–raise exception
CS550: Advanced Operating Systems
26
• Time Out
–Kernel starts timer when request sent
–If timer expires, resend message
–If message was lost - server cannot tell the
difference
–If message lost too many times ==> “cannot
locate server”
CS550: Advanced Operating Systems
27
• More difficult to handle
• Rely on timer again?
• Problem: Client’s kernel doesn't know why no
answer!
• Must distinguish between
–request/reply got lost?
–server slow
• Why?
CS550: Advanced Operating Systems
28
• When server crashes?
– After execution
– After receiving message but BEFORE execution
• Solutions:
– Wait until server reboots (or rebind)
– Give up immediately and report failure
– Guarantee nothing
– “exactly once semantics”
CS550: Advanced Operating Systems
29
• Client sends a request and crashes
–Computation active - but no parent active
–unwanted computation called “orphan”
• Orphan’s can create problems:?
• Solutions:
–
–
–
–
Extermination:
Reincarnation:
Gentle reincarnation:
Expiration:
CS550: Advanced Operating Systems
30
CS550: Advanced Operating Systems
31